DESCRIPTION: Recent studies report elevated rates of HIV, sexually transmitted infections, and substance use among veterans. Many of the 1.9 million new veterans returning from Afghanistan and Iraq (Operations Enduring Freedom and Iraqi Freedom; OEF/OIF), especially those veterans who have been exposed to combat, will engage in high risk sexual behaviors. The relationship between risk-taking and combat exposure may be related to PTSD, as veterans with PTSD endorse even higher rates of risk-taking. There are elaborate but poorly-supported theories that sexual risk-taking may represent either an effort to cope with PTSD symptoms, a consequence of substance-related risk-taking, and/or an association with other comorbidities such as impulsivity (1). A study using ecological momentary assessment (EMA) is proposed to clarify these antecedents. Because OEF/OIF veterans PTSD is complicated by overlapping comorbidities, such as impulsivity and substance use, standard retrospective summary data cannot clarify the antecedent mental states and behaviors that are associated with engaging in HIV-related sexual risk behavior (HSRB). Standard retrospective estimates are also imprecise because they are derived by heuristics that may rely on non- representative events or time periods to integrate 28 days of information. EMA can be used to understand the nature of the association between PTSD and HSRB by measuring the associations as they occur in real-time. Using EMA, this project will estimate any systematic biases that occur in retrospective measurement of these behaviors and states, and determine if data collected in situ clarify more determinative models of HSRB. In the proposed study, after a four-person pre-pilot stage, 36 OEF/OIF veterans reporting recent PTSD symptoms and recent HSRB will complete baseline assessments in multiple domains, and then will be called three times daily for the next 28 days to complete brief, interactive voice response (IVR)-delivered assessments of time- specific PTSD symptoms, substance use, mood, impulsivity (using a recently-developed assessment of momentary impulsivity designed specifically for momentary sampling), and HSRB. Veterans will complete a follow-up in-person assessment that, like the baseline assessment, will include 28-day retrospective measures of the same domains that were measured by EMA, and a brief review of their 28-day EMA data to determine their own interpretations about changes in symptoms over time, and factors associated with HSRB occurrence. In summary, ecological momentary assessment will be used to clarify immediate, and potentially causal, influences on HSRB in at-risk veterans whose engagement in HSRB is not well understood.